usb: gadget: configfs: Create control_config group

Control_config is a group under gadget that acts as a normal config group, except it does not appear in cdev->configs.

Functions that have exactly zero descriptors can be linked into control_config. These functions are bound and unbound with the rest of the gadget, but are never enabled. Also, functions with zero descriptors cannot be used in real configs.

Create configfs_setup(), which will first attempt composite setup. If that fails, it will go through functions in control_config and use req_match to find and deliver the request to a function that can handle it.

This allows the user to create a functionfs instance dedicated to handling non-standard control requests no matter what functions or configurations are currently active.

Signed-off-by Jerry Zhang <[email protected]>